1. Field of the Invention
The present invention relates generally to improvements in a device for handling stacked sheets of thin materials, such as sheets of paper. More particularly, the present invention relates to a high-speed mechanism for removing single sheets of paper from the top of a stack of sheets, where such sheets are transported for individual processing, such as by optical scanning, after being removed from the stack.
2. Description of the Prior Art
The use of rotating friction rollers or belts to "pick", that is to engage and remove laterally, the top sheet in a stack of sheets is known. A common problem with such mechanisms is that they do not always successfully pick a single sheet from the top of the stack, because the sheet below the top sheet tends to adhere to the lower surface of the top sheet and to follow the top sheet as it moves. In some circumstances, more than one sheet will follow the top sheet. If more than one sheet is removed from the stack, this can cause severe problems, particularly in machines that scan information contained on both sides of a sheet, e.g., a two-sided, optically scanned answer form. In higher speed machines that handle hundreds or thousands of documents in sheet form per hour, even occasional malfunctions of the sheet picking mechanism can lead to numerous errors or cause significant losses in throughput. Ordinarily, to avoid errors, immediate multiple-sheet detection must occur and operator intervention will be needed to remove multiple-picked documents and restore them to the document stack. If multiple-picked documents are not recognized, jams and/or misreading of documents can occur. The greater the picking and processing speed of the equipment, the greater the throughput loss when an operator must intervene.
Detection of multiple-picked sheets and mechanisms to correct multiple picking, such as reverse or stripping rollers, are known in the prior art. But such mechanisms are complex, less than perfectly reliable and only useful to correct the problem of a multiple pick once it has occured. The present invention recognizes that it is preferable to avoid multiple picking in the first place. Accordingly, it focusses on correcting the reasons why multiple picking occurs in conventional picking mechanism designs and proposes a novel picking mechanism that is intended to significantly reduce the likelihood of multiple sheet picks.
In particular, a study of the prior art reveals that in prior art sheet picking mechanisms the pick roller or pick belt rests on the top of the sheet stack with the top sheet in a tangent position relative to a curved surface of a pick roller or belt. To obtain adequate frictional force to draw the top sheet from the stack, the pick roller or belt exerts a normal force on the top sheet (perpendicular to the plane of the sheet), which is transmitted down through other sheets. Due to the resilience and flexibility of stacks of paper and the friction surface of the pick roller or belt, this normal force spreads to form a compression area around the line along which the pick roller or belt contacts the top sheet in a tangent relationship. This spreading creates an area of friction force between the top sheet and the sheet next below. Similar between-sheet forces extend further into the stack, lessening with distance from the top sheet. It is this between-sheet frictional force area that causes sheets below the top sheet to tend to follow the top sheet when it is drawn laterally from the sheet stack.